The disclosure relates in general to transconductance filter circuits. In particular, the disclosure relates to a transconductance filter circuit for digital video disk (DVD) systems.
FIG. 1 is a circuit diagram of a conventional GM-C filter. A conventional GM-C filter comprises a transconductor GM and two capacitor arrays 10. In practice, frequency banks of the GM-C filter are switched using capacitor switches S1˜Sn. For example, read signals generated by a read circuit of a disc drive are in different frequency banks when the disc drive operates at different speeds. In FIG. 1, capacitor switch circuits S1˜Sn are arranged in an array, connected between output terminals Vo+ or Vo− of transconductor GM and a ground level. Switch transistors M of capacitor switch circuits S1˜Sn are respectively turned on or turned off according the logic level of bit control signals V1˜Vn. Capacitance of capacitors C1˜Cn are respectively c, 2c, 4c, . . . , 2nc. Capacitors with larger capacitance are larger. As capacitor switch circuits controlled by most significant bits (MSB) of a set of control signals are turned off, stray capacitance is larger due to W=Gm/C. The stray capacitance generated by switch transistors limits the range of the operating frequency bank of the conventional transconductance filter circuit.
FIG. 2 is a circuit diagram of another conventional GM-C filter. The difference between FIG. 2 and FIG. 1 is that the capacitor arrays 10′ are connected between output terminals Vo+ or Vo− of transconductor GM, capacitance of capacitors C1′˜Cn′ in capacitor switch circuits S1′˜Sn′ are respectively c/4, 2c/4, . . . , 2nc/4. As the capacitance of capacitors are ¼ of that in FIG. 1, the total size of the capacitors in FIG. 4 is relativly smaller. In addition, the equivalent capacitance of each capacitor switch circuit S1′˜Sn′ is the same as that of corresponding capacitor switch circuits S1˜Sn. Using capacitor switch circuit S1′ as an example, the equivalent capacitance of two capacitors C1′ with capacitance c/4 connected in parallel is c/2, which is the same with that of two capacitors C1 with capacitance c connected in serial in FIG. 1. As the capacitor switch circuits controlled by least significant bits (LSB) of control signals is turned on, the output of capacitor switch circuits is influenced by the stray capacitance generated by bottom metal-dielectric-metal BMIM, deteriorating frequency response in high frequency operation.